1. Field of the Invention
This invention relates to a process for producing ethylene-.alpha.-olefin copolymers using a novel Ziegler catalyst system. Particularly, this invention relates to a process for producing ethylene-.alpha.-olefin copolymers at a temperature higher than 120.degree. by using a polymerization catalyst comprising a titanium amide compound and an oxygen-containing alkylaluminum compound. More particularly, this invention relates to a process for producing ethylene-.alpha.-olefin copolymers narrow in composition distribution, high in molecular weight and excellent in weather resistance, colorizability, corrosion resistance and dynamic properties by using a novel catalyst system.
2. Description of the Prior Art
Olefin copolymers are used in very many fields such as film, laminate, electric wire coating, injection molded products, special molded products, and the like. It is generally known in these fields that a product excellent in transparency, impact resistance and blocking resistance can be obtained by using a polymer narrow in molecular weight distribution or composition distribution. Particularly in the case of copolymers, the molecular weight distribution and composition distribution exercise an increasing influence upon the properties of olefin copolymer as the content of copolymerized olefin increases. Thus, an olefin copolymer narrow in molecular weight distribution and composition distribution is waited for.
As a process for producing olefin copolymers, the method of using the so-called Ziegler-Natta catalyst comprising a transition metal of Group IV to VI of the periodic table and an organometallic compound of a metal of Group I to III is generally widely known.
As the process for producing an olefin copolymer at high temperatures by using these Ziegler type catalyst, the following two processes are practiced today.
The first process is usually called "solution process" which comprises polymerizing or copolymerizing olefins in a solvent such as cyclohexane or the like. In this process, an olefin is polymerized at a temperature of 20.degree. to 250.degree. C. and a pressure of 5 to 50 kg/cm2 by the use of a Ziegler type catalyst to form a solution of polymer.
The second process is usually called "high pressure ion process" which comprises polymerizing or copolymerizing olefins in the absence of solvent at a high temperature and a high pressure to form a polymer in a molten state.
These high temperature solution polymerization process and high pressure ion polymerization process using a Ziegler catalyst are known to have a merit that the reactor is compact and comonomer can be selected with a high degree of freedom.
On Ziegler type solid catalyst for use at high temperature, a variety of improvements have been proposed up to date as described in, for example, Japanese Patent Application KOKAI (Laid-Open) Nos. 51-144397, 54-52192, 56-18607, 56-99209, 57-87405, 57-153007, 57-190009 and 58-208803. However, all these techniques give a polymer broad in composition distribution and are unsatisfactory in transparency and dynamic properties of the formed polymers.
On the other hand, as a method for obtaining an olefin polymer narrow in molecular weight distribution and composition distribution, a process for polymerizing an olefin by using a catalyst formed from a vanadium type catalyst component and an organoaluminum compound catalyst component is known. However, this process is disadvantageous in that activity per transition metal is low and the activity further decreases when polymerization is carried out at a high temperature of 120.degree. C. or above.
In order to solve such a problem, processes using a catalyst system comprising a titanium compound or zirconium compound and an aluminum compound have been disclosed up to today, and recently, a process using a catalyst system comprising a titanium compound or zirconium compound and aluminoxane has been proposed [Japanese Patent Application KOHYO (International Laid-Open) No. 1-503788; Japanese Patent Application KOKAI (Laid-Open) No. 62-121708].
However, when such a catalyst system is used in the high temperature solution polymerization process, the formed copolymer is low in molecular weight and cannot be said to be satisfactory in practical properties. Further, such a catalyst system is not sufficient in the ability to copolymerize .alpha.-olefins. Thus, expensive .alpha.-olefin must be fed into the polymerization system in a large amount, which is undesirable in economical point of view.
As processes for homo- or co-polymerizing olefins by using a catalyst system comprising a compound having titanium-nitrogen bond and an organoaluminum compound, a process using a catalyst system comprising a solid component prepared by supporting a titanium amide compound on magnesium halide and an organoaluminum compound (EP-A-0 320169; Italian Patent No. 867243), a process using a catalyst system comprising a titanium diphenylamide compound and an organoaluminum compound [EP-A-0 104374; Japanese Patent Application KOKOKU (Examined Publication) No. 42-11646], a process using a catalyst system comprising an aryl substituent-containing titanium amide compound and an organoaluminum compound [Japanese Patent Application KOKOKU (Examined Publication) No. 42-22691], and a process using a catalyst system comprising a lower alkyl-containing titanium amide compound such as dimethylamidotitanium trichloride and the like and an organoaluminum compound [J. of Polym. Sci. Part A-1, 241, 6 (1968)]have been proposed.
However, if a copolymerization of ethylene and .alpha.-olefin is carried out with the catalyst systems disclosed in these papers, no satisfactory result has been obtained. Thus, according to the process disclosed in EP-A-0 320169 and Italian Patent No. 867243, the formed ethylene-.alpha.-olefin copolymer has a broad composition distribution. According to the processes disclosed in EP-A-0 104374, Japanese Patent Application KOKOKU (Examined Publication) Nos. 42-11646 and 42-22691 and J. Polym. Sci. Part A-1, 241, 6 (1968), catalyst activity, copolymerizability and narrowness of composition distribution are unsatisfactory.
In order to solve the above-mentioned problems, the present inventors previously proposed [Japanese Patent Application KOKAI (Laid-Open) No. 2-77412]a process for copolymerizing ethylene and .alpha.-olefin to give a copolymer having a narrow composition distribution which comprises using a catalyst system comprising an organoaluminum compound and a liquid catalyst component comprising a titanium compound represented by the following general formula: EQU (R.sup.1 R.sup.2 N).sub.4-(m+n) TiX.sub.m Y.sub.n
wherein R.sup.1 and R.sup.2 each represent a saturated hydrocarbon group having 8 to 30 carbon atoms, X represents a halogen atom, Y represents an alkoxy group, m is a number satisfying 1.ltoreq.m.ltoreq.3, and n is a number satisfying 0.ltoreq.n.ltoreq.2, and (m+n) satisfies 1.ltoreq.(m+n).ltoreq.3.
However, this process had a fault that, if the catalyst system was used at a high temperature, the catalyst activity was very low and the ability to copolymerize .alpha.-olefin was low, and the resulting polymer was unsatisfactory in composition distribution.